The present invention relates generally to optical fiber connectors and a method for orienting the optical fiber within the bore of a ferrule of a fiber optic connector to create a population of ferrules and connectors with improved mating characteristics.
Optical fibers are widely used in a variety of applications, including the telecommunications industry. For example, optical fibers not only serve as a medium for long distance signal transmission, but are being increasingly routed directly to the home or, in some instances, directly to the desk or another work location.
With the increasing use of optical fibers, an efficient means of coupling fibers, such as to other optical fibers, to a patch panel in the central office or office building, or to various remote terminals or pedestals is required. However, in order to efficiently couple the signals transmitted by the respective optical fibers, an optical fiber connector must not significantly attenuate or alter the transmitted signals.
A number of factors affect the performance of mated optical fiber connectors. These include surface finish of the optical fiber in the connector and the alignment of the optical fiber of one connector with the optical fiber of a second connector and, more specifically, the alignment of the cores of the respective fibers. The loss or attenuation of an optical fiber with a 50 micrometer core diameter due to surface reflection is about 0.3 dB and is also about 0.3 dB due to offset, or lateral displacement. See Y. Koyama et al., Development and Application of Precision Optical Fiber Connectors, Bull. Japan Soc. of Prec. Eng., Vol. 15, No. 3 (Sep. 1981). Various methods, including mechanical polishing alone or in combination with laser polishing as are well-known in the art, have been developed to improve the surface finish of the fiber, thereby reducing attenuation as shown, for example, in U.S. Pat. No. 5,317,661 to Szentesi et al. Developing efficient and economical methods to minimize lateral displacement of mated fiber cores with respect to each other has been, however, more elusive.
Lateral displacement or offset of fiber cores of mated connectors results from the contribution of three factors: (1) eccentricity of the ferrule bore with respect to the ferrule""s outside surface, (2) eccentricity of the fiber core relative to the fiber cladding, and (3) differences in the outside diameter of the fiber and the inside diameter of the bore of the ferrule in which the fiber is set. While the quality of ferrules has improved over time, ferrules are not perfectly concentric, i.e., the central axis of the cylindrical bore does not coincide with the central axis of the outside surface of the ferrule. Furthermore, the degree of eccentricity varies in a large population of ferrules, some being more eccentric, i.e., less concentric, than others. Similarly, the inside diameter of the ferrule bores in a large population of ferrules varies within certain limits, about 125.0-126.0 xcexcm, so that the fiber does not fit tightly within the ferrule bore, thereby adding to the overall lateral displacement of the core of the optical fiber relative to the outside of the ferrule. Of three factors discussed above, the eccentricity of the fiber core relative to the cladding is the least significant contributing factor to overall eccentricity of the fiber optic connector.
An early method of minimizing overall connector eccentricity, i.e., centering the fiber within the ferrule, is disclosed by N. Suzuki et al., xe2x80x9cA New Demountable Connector Developed For A Trial Optical Transmission System, p. 351-354, IOOC 1977. This method involved minimizing the eccentricity of connectors comprised of a fiber, a stainless steel plunger, a glass tube, and a stainless steel pivot. The plunger is machined and polished to a outside diameter of 2.499 mm with a 0.001 mm accuracy and a roundness of 0.6 microns. Unlike ferrules presently being used, however, the plunger has a large bore for receiving the stainless steel pivot. The optical fiber is inserted in a glass tube filled with epoxy resin. The glass tube is then inserted into the stainless steel pivot which is also filled with epoxy resin. After the epoxy is cured, the pivot is polished. Using a microscope, TV camera and monitor, V groove or V block, micromanipulator, and dummy plunger (alignment jig), the fiber is centered inside the plunger with respect to the outside surface of the plunger. First, the jig is placed in the V groove and positioned using the micromanipulator so that by rotating the jig, markers on the jig trace concentrically along a target circle drawn on the screen. The connector plunger then replaces the dummy plunger in the V groove, and the pivot containing the optical fiber is inserted into the plunger. The pivot is then moved by the micromanipulator until the core of the fiber is centered in the target circle on the monitor, thereby positioning the fiber at the center of the plunger. This method is expensive, time consuming, and not suitable for mass production of connectors.
Another example is U.S. Pat. No. 4,880,291, entitled xe2x80x9cOptical Fiber Connector and Methods of Making.xe2x80x9d That patent discloses two ferrules to be used in a connector that are provided from contiguous portions of the same tubular preform. Prior to separation of the ferrules from the preform, the free end portion of each ferrule (called plugs in the patent) is mounted in a connector body. The connectors are provided with a tab and the connectors are attached to the ferrules so that tabs are aligned longitudinally. As a result, the end faces of the ferrules that were once contiguous can then be aligned longitudinally when mated and the fibers in the ferrules will also be aligned. However, this method does not allow for alignment of ferrules that are not from the same preform. These connectors also require that the installers keep the connectors paired to each other. If one connector is bad or must be changed, then both connectors must be discarded/changed. This increases not only the costs, but the number of paired connectors the installers must carry at each job site.
Other methods have attempted to center the optical fiber within the ferrule differently. For example, one method, disclosed by G. Khoe in xe2x80x9cSingle-Mode Fiber Connector Using Core-Centered Ferrulesxe2x80x9d, IEEE Transactions On MicroWave Theory And Techniques, Vol. MT-30, No. 10, October 1982, is to first glue the fiber into a ferrule without regard to the position of the fiber relative to the outer surface of the ferrule. A special lathe, comprising a micromanipulator, an optical alignment system, and a diamond cutter, is then used to cut down the ferrule outer surface until the fiber is centered within the ferrule. Again, this method is not cost-effective, nor suited to mass production.
More recent assembly methods have focused on assembling the optical fiber within the ferrule, and then orienting the resulting total offset relative to a key on the ferrule. Because the eccentricity errors are positioned randomly, these errors can result in very high total offset, i.e., the eccentricity of the ferrule bore relative to the outer surface of the bore and the eccentricity of the optical fiber relative to the bore may be oriented in the same direction resulting in a large offset of the fiber relative to the outer surface of the ferrule. The attenuation of the signal introduced by the offset is somewhat reduced when mating the connector with a second connector by mating the connectors in various orientations until the lowest attenuation is achieved. This process is expensive and is limited in effectiveness by the size of the resulting offset distribution. The typical connector alignment hardware is capable of only coarse adjustments (typically 90xc2x0 to 180xc2x0) that make it difficult and expensive to obtain maximum performance from the fiber optic connector. Furthermore, the process of determining the offset by repeated mating and de-mating results in wear and tear on the connector.
Alternatively, the concentricity tolerance of the bore of the ferrule may be determined with respect to the outer surface of the ferrule. Concentricity tolerance (referred to hereinafter as xe2x80x9cconcentricityxe2x80x9d) is the diameter of the cylindrical tolerance zone within which the axis of the features, in this case the central axis of the ferrule bore, must lie. See L. Foster, Geo-Metrics II 292-97 (1986); 4 Tool and Manufacturing Engineers Handbook, chapt. 4 (C. Wick and R. Veilieux eds. 1987). The measure of concentricity is therefore twice the measure of eccentricity. Once the concentricity of the bore relative to the ferrule is determined, the direction of concentricity is determined and marked with a key, thereby allowing connectors to be mated with the concentricity in the same direction, i.e., reducing offset of the cores of the fibers in the connectors.
Therefore, while methods exist for centering the optical fiber in a ferrule, they are slow, expensive, and not suitable for use in an assembly line used to mass produce connectors. As a result, no cost effective, commercial process presently exists that centers the fiber relative to the outer surface of the ferrule of a connector. The present method of orienting the concentricity, or offset of the fiber with respect to the ferrule surface, is not satisfactory because it does not correct or eliminate the concentricity but merely attempts to orient the concentricity of each ferrule with respect to a key so that the concentricities of mated ferrules are oriented in the same direction to minimize the offset of the fiber cores to the greatest extent possible. Because the magnitude of the concentricities or eccentricities can vary significantly from ferrule to ferrule, simply orienting the concentricities in a known direction does not permit the optical fiber to readily align with the optical fiber of another connection. Thus, the current methods do not meet the increasing demands for connectors having minimal attenuation losses.
Accordingly, the present invention is directed to low-loss intermatable ferrules that substantially obviate one or more of the problems due to the limitations and disadvantages of the related art.
To achieve this and in accordance with the purpose of the invention, as embodied and described, the invention is directed to a method of making a population of intermatable ferrules, each of the ferrules being mounted on an optical fiber, and each ferrule having an outer surface and defining a longitudinal bore, comprising the steps of selecting from a first set of ferrules a second set of ferrules with a bore having a predetermined eccentricity and a predetermined inner diameter, the eccentricity of the bore relative to the outer surface of each of the ferrules, providing an optical fiber, the optical fiber having an outer diameter smaller than the inner diameter of the ferrule bore, and positioning an optical fiber in the bore of each of the ferrules on the basis of the eccentricity so the fiber is generally concentric relative to the outer surface of the ferrule.
In another aspect, the invention is directed to a population of independent, intermatable cylindrical ferrules including a plurality of cylindrical ferrules having an outer surface and a longitudinal bore, the bore having an inner diameter, a center point, and a cylindrical wall of varying thickness between the bore and the outer surface; and an optical fiber having a core and a cladding, the core having a center and the cladding having an outer diameter, the optical fiber mounted in the bore of each of the ferrules against wall with a portion having the greatest thickness, whereby the center of the optical fiber core is positioned within a predetermined range of the center of the ferrule.
In yet another aspect, the present invention is directed to a population of independent, intermatable ferrules that includes a plurality of ferrules, each of the ferrules having an outer surface and a longitudinal bore, the bore having a predetermined inner diameter and an eccentricity relative to the outer surface, an optical fiber mounted in the bore of each of the ferrules generally concentric relative to the outer surface of the ferrule, whereby when any two ferrules of the population of ferrules are mated in a connector sleeve, centers of the optical fibers are offset less than a predetermined value relative to one another.
In yet another aspect, the present invention is directed to a population of independent, intermatable ferrules including a plurality of ferrules having an outer surface and a longitudinal bore, the bore having an eccentricity relative to the outer surface, a optical fiber mounted in the bore of each of the ferrules generally concentric relative to the outer surface of the ferrule, whereby when any two ferrules of the population of ferrules are mated in a connector sleeve, the mating point has a signal loss less than a predetermined value.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.